Method for realizing surface acoustic wave filter arrangements and filter arrangement obtained in accordance to this method

ABSTRACT

The invention relates to a method for realizing surface acoustic wave filter arrangements of the kind comprising a piezo-electric substrate and on said substrate metallic strip elements constituting impedance filter elements arranged in a network scheme. In such an arrangement by electrostatic interaction between the network elements is produced a parasitic resonance at a frequency different from the operational resonance frequency. The method is characterized in that means are provided in the filter arrangement which produce a shifting of the parasitic resonance frequency to a frequency substantially equal to said operational resonance frequency. The invention can be used for filter arrangements.

The invention relates to a method for realizing surface acoustic wave(SAW) filter arrangements, of the kind comprising a piezo-electricsubstrate and on said substrate metallic strip elements constitutingimpedance filter elements arranged in a network scheme and a surfaceacoustic wave filter arrangement obtained in accordance to this method.

BACKGROUND OF THE INVENTION

Surface acoustic wave filter arrangements of this kind are alreadyknown. The FIG. 1 shows a portion of such a network realized in theshape of a ladder network, the circuit diagram of which is shown on FIG.2. On this figure, between an input terminal 1 and an output terminal 2a plurality of seriesly connected impedance elements 3 are shown with aparallel impedance element 4 between respectively the input terminal 1,the output terminal 2 and the nodes 5 between two adjacent seriesimpedance elements 3 on the one hand, and the ground 6 on the otherhand. On the FIG. 1, the series impedance elements 3 are realized asupper and lower metallic strips forming bus-bars 7, 8 deposited on apiezoelectric substrate (not shown). The parallel impedance elements 4are realized as inter-digital electrodes 9, 10 extending perpendicularlyrespectively from the upper bus-bar 7 and the lower bus-bar 8.

The FIGS. 3 and 4 illustrate the electrical behaviour of a laddernetwork in accordance to the FIGS. 1 and 2. FIG. 3 shows the simulatedimpedance performance Zi/Zo of impedance elements 3 and 4, Zi and Zobeing the input and output impedances. The first impedance element 3 hasa low impedance of for instance 1 Ohm, at a resonance frequency f₁ and ahigh impedance of for instance 1000 Ohms, at an anti-resonance frequencyf₂. The second impedance element is shifted in frequency so that theresonance frequency f₁ of the series impedance element 3 and theanti-resonance frequency f₄ of the parallel impedance element 4 wereabout the same.

The simulation of this impedance element arrangement yields theelectrical bandpass filter performance such as shown on FIG. 4 for aΓ-type scheme with a passband around f₁ and f₄ and deep notches at f₂and f₃. This figure shows the dependency of the amplitude AM in dB fromthe frequency f in MHz.

In practice the real impedance performance differs from the illustratedsimulated performance, particularly due to parasitic effects. One ofthese is caused by the electrostatic interaction between the bus-bars 7and 8 and the edges 12 of the inter-digital electrodes 9 and 10. Thiselectrostatic interaction creates a periodic charge and electric fielddistribution in the bus-bar area 13 adjacent the correspondinginter-digital electrodes edge 12 and in the gap 14 with the sameperiodicity as for inter-digital electrodes 9, 10. On the FIG. 4 theelectric field distribution is illustrated by arrows. Both the gapelectrical field and the bus-bar charge distribution create a parasiticacoustic resonance on a higher frequency, the frequency shift and theamplitude of which depend on substrate material, metal thickness,mark-to-period ratio and is usually in a range of 0.01 to 1% for thefrequency shift and in a range of 0.1 to 10% for the amplitude. Due tothis parasitic interaction the real passband of an impedance elementfilter is narrower than the simulation made without taking into accountthe parasitic interaction.

The FIG. 5 illustrates the frequency response FR of a 947.5 MHz surfaceacoustic wave filter whereon the dashed line indicates measured valuesand the solid line the simulated filter response. It is to be noted thatthe 3 dB level L of the measured curve has a width of 5 MHz less thanthe simulated curve, on the lower frequency side of the centrefrequency. In the filter which has been used the parallel impedanceelements 4 were about four times longer than the length of the elements3. When the length of the series impedance elements 3 is greater thanthe length of the parallel elements 4, the main difference between themeasured and the simulated curve is on the side of the higherfrequencies with respect to the centre frequency of the passband.

A second parasitic effect resides in the wave-guide mode excitation.

BRIEF SUMMARY OF THE INVENTION

The main object of the invention is to propose a surface acoustic wavefilter which does not have the inconveniences which have been describedfor the known-filter arrangements.

For reaching this object the method for realizing a surface acousticwave filter arrangement according to the invention is characterized inthat means are provided for shifting the parasitic resonance created byelectrostatic interaction between the inter-digital electrodes and thebus-bars to a frequency substantially equal to the frequency of theoperational resonance of the filter arrangement.

The surface acoustic wave filter arrangement for putting into practicethis method is characterized in that for obtaining said parasiticresonance frequency shift, additional cut outs are provided in thebus-bar areas adjacent to the ends of the inter-digital electrodes atperiodically succeeding locations in the longitudinal axis of thesebus-bars.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in a more detailed manner with furtherobjects and advantages in the following description referring to theannexed figures where

FIG. 1 shows a portion of a known surface acoustic wave filterarrangement having an inter-digital structure;

FIG. 2 illustrates the circuit diagram of an inter-digital surfaceacoustic wave filter arrangement in accordance to the FIG. 1;

FIG. 3 is a diagram showing the impedance performances of thearrangement shown on FIG. 2;

FIG. 4 is a diagram showing an amplitude performance of the Γ-typeladder network of FIG. 2;

FIG. 5 illustrates the amplitude performance of a 947.5 MHz surfaceacoustic wave filter arrangement showing by a dot line the measuredamplitude performance and by a solid line curve the simulated amplitudeperformance;

FIG. 6 is a schematic diagram of a portion of a surface acoustic wavefilter arrangement in accordance to the invention;

FIG. 7 is a schematic diagram of another embodiment of a portion of aninter-digital surface acoustic wave filter arrangement in accordance tothe invention;

FIG. 8 is a plot of the passband of a 860 MHz inter-digital surfaceacoustic wave filter arrangement in accordance to the invention, asfunction of holes length;

FIG. 9 is a diagram showing by a dot line curve the amplitudeperformance of a standard 860 MHz surface acoustic wave filterarrangement and by a solid line curve the amplitude performance of acorresponding filter arrangement according to the invention;

FIG. 10 is a schematic drawing of an inter-digital structure surfaceacoustic wave filter arrangement with dummy electrodes;

FIG. 11 is a schematic drawing of an inter-digital structure surfaceacoustic wave filter arrangement with dummy electrodes in accordance tothe invention;

FIG. 12 is a diagram showing the surface acoustic wave velocitydistribution across the aperture of the filter arrangement according toFIGS. 10 and 11;

FIG. 13 is a schematic drawing of another embodiment of a inter-digitalstructure surface acoustic wave filter arrangement with dummyelectrodes.

DETAILED DESCRIPTION OF THE INVENTION

The invention is based upon the discovery that the inconvenience of theparasitic effects due to the electrostatic interaction between thebus-bars 7, 8 and the edges 12 of the inter-digital electrodes 9, 10 inan inter-digital structure surface acoustic wave filter arrangement suchas shown on FIG. 1 can be eliminated by providing means in such astandard filter arrangement, producing a frequency shift of theparasitic resonance to a frequency substantially equal to theoperational resonance frequency of the inter-digital structure anddecreasing the influence of parasitic responses.

The FIG. 6 shows an inter-digital structure surface acoustic wave (SAW)filter arrangement proposed to this purpose by the invention. On thisfigure the reference 16 indicates an inter-digital surface acoustic wavefilter structure portion constituting the transducer part of the filterarrangement, to which is associated a reflector input portion 17 in anon per se manner.

In accordance to the invention, the means for shifting the parasiticresonance in the way to coincide with the operational resonancefrequency of inter-digital structure portion 16, i.e. for synchronisingboth resonances, reside in providing holes 19 in the upper and lowerbus-bars 7 and 8. For having the best synchronisation of bothresonances, the holes 19 have the shown rectangular shape. The holesperiod p, i.e. the distance between two vertical centre lines of twosuccessive holes in the direction of the SAW propagation should besubstantially equal to the inter-digital electrode pitch P. The mark toperiod ratio of the inter-digital electrodes M/P should be substantiallythe same as the mark to period ratio m/p of the holes, where M is thewidth of an interdigital electrode 9, 10 and m the distance between twoadjacent holes.

In this case, the bus-bar holes 19 create a periodic grating whichincreases the effective coefficient of reflection of the impedanceelements and, accordingly, the quality (Q) factor of these elements isheigher, because the SAW energie propagates not only in the transduceraperture area TA defined by the overlap distance of adjacentinter-digital electrodes 9, and 10 such as shown on FIG. 6.

When, in accordance to the invention, the centre frequencies of both theparasitic resonance and the operational resonance are substantially thesame, the total impedance has the requested form which does not changethe impedance element filter performance. It takes into account onlythat the effective aperture is a little higher than the real overlap TAof the electrodes. Obviously, to achieve the best results, the bus-barto electrode edge gap 14 and the separation 20 between a bus-bar edge 13and the adjacent hole edge 21 should be as less as possible.

Furthermore, for the best synchronisation of both resonances, the centreof the distance between two adjacent holes 19 for both bus-bars 7, 8should be substantially on line with the centre of the inter-digitalelectrodes. In this case, the phase of the reflected surface acousticwaves in the inter-digital electrodes and in the additional gratingcreated by the holes 19 are the same.

The advantage of this feature is in a reduction of resistance of theelectrodes of the impedance elements. It has been found that thepassband varies in function of the length l of the holes in thedirection perpendicular to the SAW propagation.

The FIG. 8 illustrates this dependency, the bandpass width at the levelof 2.5 dB in MHz being indicated on the ordinate whilst the lengthdefined by the ratio 1/P is indicated on the abscisse, P being theinter-edge electrode pitch (FIG. 6). The FIG. 8 shows the result of thepassband measurements for 860 MHz realized on a 42° Lithium Tantalatsubstrate. It follows from the shown plot that the holes 19 are mostefficient when their length l is in the range of 4 to 7 P. Using holeshaving a length greater than 20 P is not effective due to the increasein size and additional insertion loss associated with high electroderesistance. Using the smallest holes at a length l less than 0.4 P isalso not effective.

FIG. 9 shows the measured performances of a SAW filter arrangement withadditional holes 19 in accordance to the invention and the performancesof a standard filter arrangement. The performances of the filterarrangement according to the invention are shown by a solid line curve,the length l of the holes having the value 7 P. The standard filterperformance is shown by the dot line curve. It results from a comparisonof both curves, that the invention provides a better shape factor designbecause the passband is wider in comparison to the standard filtercurve, the outband rejection being the same.

The FIG. 7 illustrates an embodiment of an inter-digital surfaceacoustic wave (SAW) filter arrangement according to the inventionwherein the length l of the holes 19 is variable. Due to the variationof the whole length, the parasitic response due to parasitic wave guidemodes can be decreased. The lower sensitivity to parasitic wave guideresponses is known in the field of SAW filters for apodized transducershaving inter-digital electrodes with variable length. The inventiondiffers therefrom by the fact that the proposed structure has unapodizedinter-digital electrodes with a constant overlap but wherein the holelength l, i.e. the passive part of the transducer, is variable. Thevariation of the hole length can be made in the way to have the shownsmooth shape with an increasing and then decreasing length in thedirection of the propagation of the waves. A smooth variation shapesimilar to cos(x)^(n) is preferable with n=1, 2, 3 . . . , and x in therange from −π/2 to +π/2.

In the foregoing it has been written that to achieve best results insynchronising the parasitic resonance and the operational resonance ofthe inter-digital structure the bus-bar to electrode edge gap 14 and theseparation 20 between a hole edge 20 and the corresponding bus-bar edge13 (FIG. 6) should be as less as possible. When making the separation 20zero, a dummi finger inter-digital structure is obtained such asillustrated on FIGS. 10 and 11 with a constant overlap or transduceraperture TA, the inter-digital space portion 25 formed by the holes 19with the separation 20 having become 0 constituting the passive part ofthe transducer.

On the FIGS. 10 and 11 the portion 25 has a constant length. In theembodiment shown on FIG. 13, the length l′ of the portion 25 varies inthe wave propagation direction according to a smooth shape preferablysimilar to cos(x)^(n) where n=1, 2, 3 . . . , and x in the range of −90/2, +π/2.

In the dummi finger structures shown on FIGS. 10 and 11 created by holeswith the separation 20 having become 0 the electrode gap 27 is less thanthe electrode width. The minimum gap size is limited by process and DCvoltage and RF power requirements. When the gap area is smaller, theamplitude of parasitic response is smaller too.

In the dummi electrode structure case illustrated on FIG. 11, theinter-digital electrode edges 29, 30 form an angle α with the edges ofthe bus-bars which is in the range of 0 to 60°. This configuration hasthe advantages of reduction of the transition area and thesynchronization of resonances, which are explained by the FIG. 12showing the surface acoustic wave velocity distribution V across theaperture direction A. The smallest phase velocity V₀ corresponds to theinter-digital area, the highest velocity V_(G) to the free surface area,the phase velocity on the bus-bar area V_(BB) being therebetween. In thecase of the angled gap shown on FIG. 11, the phase velocity V_(α) is, incomparison to the velocity V_(G) relatively small so that the velocityvariation across the aperture is very small.

It is furthermore to be noted, with reference to the FIG. 6, that thetotal grating aperture GA is overlapping the inter-digital electrode andholes apertures. Due to this fact, the effective coefficient ofreflection of the impedance elements and, as results, the Q-factor ofthe impedance elements is higher because more SAW energie comes back inthe inter-digital structure.

It is furthermore to be noted that the best synchronisation of theparasitic and operational resonances can be obtained with a SAWimpedance element filter arrangement having additional sub-layers on thesurface of the bus-bars adjacent to the inter-digital structure whichcreates the same SAW velocity as in the inter-digital structure.

It is obvious that the invention is not restricted to the filterarrangements which have been described and illustrated for examplepurposes only but relates to all filter arrangements where similarparasitic resonance phenomena are produced.

1. A surface acoustic wave filter comprising: bus-bars extending in thedirection of propagation of surface acoustic waves and inter-digitalelectrodes extending perpendicularly from said bus-bars, said electrodesbeing spaced apart by an inter-digital electrode pitch P, said bus barsfurther having parasitic resonance frequency shifting cut outs formedtherein, said cut outs in the bus bars being spaced apart by saidinter-digital electrode pitch P in a parallel relationship to thedirection of the surface acoustic wave propagation; wherein the cut outscreate a dummi finger structure with electrode to electrode gaps lessthan an inter-digital electrode width M.
 2. The surface acoustic wavefilter of claim 1 wherein center lines of the cut outs in the upper andlower bus-bars align with center lines between two adjacentinter-digital electrodes.
 3. The surface acoustic wave filter of claim 1wherein the length of the cut outs in the direction perpendicular to thesurface acoustic wave propagation direction is rn the range of 0.4 to 20times the inter-digital electrode pitch P.
 4. The surface acoustic wavefilter of claim 1 wherein the length of the cut outs in the directionperpendicular to the surface acoustic wave propagation is variable. 5.The surface acoustic wave filter of claim 1, wherein the edges formingsaid gap and the surface acoustic wave propagation direction form anangle in the range of 0 to 60°.
 6. The surface acoustic wave filter ofclaim 1, wherein the inter-digital electrodes have a mark to periodratio M/P substantially equal to a mark to period ratio m/p of the cutouts.